1. Field Of The Invention
The present invention relates generally to the mixing of cement slurries, and more particularly, but not by way of limitation, to the mixing of very high density, high viscosity, cement slurries to be pumped down a well to cement a casing string in a well bore.
2. Description Of The Prior Art
In the construction of oil wells, one necessary operation is the cementing of a casing string of the oil well in a bore hole. This is accomplished by pumping a cement slurry down the casing string, or down a smaller pipe located within the casing string, and then forcing the cement upward into an annular space between the casing string and the bore hole.
For various reasons, it is sometimes desired to utilize cement slurries which are highly viscous and/or densified for this cementing operation. These slurries, which often contain saturated salt, large quantities of silica flour, gels or bentonite, and/or other thickening additives, are generally difficult to mix. They are difficult to mix because they have a tendency to entrain air, they are highly viscous, they have high surface area wetting requirements caused by large amounts of material such as silica flour, and because of chemical reactions taking place. A result is that often the slurries contain an excess of entrained air, e.g., greater than 3%.
High viscosity makes it difficult to disperse the bulk materials to obtain a uniform slurry, and increases the problem of removing entrained air. The result is that entrained air often causes difficulty in measurement and control of slurry density and causes pump priming problems. Another difficulty is that high viscosity slurries sometimes require that mixing rates be slowed to as low as 11/2 barrels per minute. Additionally, high surface area materials such as silica flour are hard to wet, and tend to create unwetted lumps, thus the resultant slurry is likely not to be fully mixed or homogeneous.
Prior art methods of mixing such slurries have relied solely upon mechanical agitation of the slurry such as with rotating blade-type agitators, recirculating pumps or the like to mix the dry cement material with water.
Also, the prior art has included the use of vibrational energy to aid in placing cement downhole in a well.
U.S. Pat. No. 4,736,794 to Bodine discloses a method for the sonic cementing of downhole well casings. While the cement is being flowed into the annulus surrounding the casing, sonic energy is transmitted to the bottom of the casing and operates to assure that cement fills the area around the casing in a uniform manner.
Additionally, in more conventional construction operations such as the pouring of concrete structures, roadways, and the like, it is well known to utilize concrete vibrators to aid in placement of the concrete to make sure that it completely fills forms. These concrete vibrators can be hydraulically, pneumatically or electrically powered and typically include an elongated vibrator head either cylindrical or square in cross section having a cross-sectional dimension on the order of two to three inches, and having a length on the order of twelve to eighteen inches.
In both the casing cementing operation of the Bodine '794 patent, and in the prior art use of concrete vibrators, the vibrational energy has been used for the purpose of placing concrete.